There is general agreement on the fact that bone marrow macrophages present a non-proliferating cell population. Using a sequential double-immunostaining technique, a morphometric analysis was performed on routinely processed bone marrow biopsies derived from 70 patients. The purpose of this study was, firstly, to determine the frequency of bone marrow macrophages in a variety of lesions and, secondly, to elucidate whether there is any proliferative activity detectable by immunohistochemical markers. Bone marrow pathology included reactive myelitis (RM), secondary aplastic anaemia (AP), AIDS-related myelopathy, primary (idiopathic) osteomyelofibrosis (OMF) and myelodysplastic syndromes (MDS). The monoclonal antibody PG-M1 which recognizes a formalin-resistant epitope on macrophages and PC10 raised against proliferating cell nuclear antigen (PCNA) were employed. For comparison with the PCNA-labelling index, the newly developed monoclonal antibody Ki-S1, which is associated with cell proliferation, was applied. In comparison with normal bone marrow, morphometric evaluation revealed a significant increase in macrophages in MDS, OMF, RM and especially in HIV-infected patients. Moreover, a positive immunostaining of single macrophages with PC10 was noted very infrequently. This rather inconspicuous PCNA labelling increased in AIDS. By contrast, Ki-S1 expression was found in none of the other pathologies studied. The prevalence of the macrophage population in certain disorders may have a multifactorial origin, such as inflammatory changes like intercurrent infections in AIDS and enhanced cell turnover in MDS as well as involvement of the complex pathomechanisms generating bone marrow fibrosis. In keeping with previous studies, the insignificant PCNA expression of macrophages should not be related to cell proliferation, but to unscheduled DNA strand repair which may be generated in the course of viral infection in AIDS.